I recently bought a replacement tape measure and after having bought it, wondered about its accuracy. I compared it to the inch-size grid on my cutting mat and the result was… shocking. So I compared it to other tape measures that I had on hand.
I suppose tape measures are like tire gauges: a random sample of tire gauges will all show different readings. With tire gauges though, as long as a tire gauge is consistent and produces repeatable readings, it’s useable: tires are not exactly precise laboratory-grade air vessels and it’d be both impossible and meaningless to measure or to know the precise, actual, pressure in a tire. With tape measures, I guess I can pick the ones that show the most similar readings and go with those, on the grounds that there’s safety in number!
Realistically, the inaccuracies don’t affect my sewing. First of all, I make few measurements that are in the 50-inch range. For instance, my outseam is in the 40-inch range so I don’t care that the readings don’t become unusably inaccurate until the 50″ marks. Secondly, I most often use a tape measure as a measuring stick and not as a tool of reference. If a measurement, e.g. the outseam, is X inches, then I’d measure a distance of X inches on the pattern or the fabric. As long as I make both measurements with the same tape measure, it doesn’t matter whether a measurement of X inches is actually X inches or not. It only matters that the distance I’m measuring on pattern or fabric is actually the outseam measurement.